Recently, a surface treatment of forming various coating films on a surface of a base material has been performed in order to give characteristics of corrosion resistance, workability, a beautiful appearance, and an insulation property to a base material such as a steel plate. In general, in the surface treatment for the base material, application liquid of slurry including solid particles of a ceramics element is sequentially supplied to the continuously traveling base material and then the supplied slurry is applied onto the surface of the base material by a roll applicator. As a result, a film of the slurry is formed as the coating film of the surface of the base material.
The roll applicator is a device which presses a roll rotating about an axis in a width direction of a base material (hereinafter, appropriately referred to as a material width direction) against slurry supplied to the surface of the base material and applies the slurry onto the surface of the base material by a pressing load of the roll to form a film (a coating film) of the slurry. As the roll applicator, for example, a double roll applicator which applies slurry onto each surface of the base material by using two rolls or a triple roll applicator which applies slurry onto each surface of the base material by using three rolls has been widely used. As particular advantages of the triple roll applicator, a coating film thickness of the slurry can be excellently controlled and a relatively beautiful surface of the film of the slurry can be obtained by the triple roll applicator. However, since maintenance of the roll applicator or process management becomes complex in accordance with an increase in the number of the rolls of the roll applicator, a production cost of the base material applied by the slurry increases. Thus, a single roll applicator which applies slurry by using one roll for each surface of the base material has been widely used in recent years.
As described above, a type which applies slurry onto the surface of the base material by using the roll applicator (hereinafter, referred to as a roll applicator type) can be effectively used to adjust an adhesion amount of slurry forming a coating film on the surface of the base material by squeezing an extra amount of the slurry excessively supplied to the surface of the base material in advance from the surface of the base material by a pressing load of the roll. As such a roll applicator type, a rubber roll having a groove carved in an outer circumferential surface may be used as the roll of the roll applicator in order to ensure a necessary adhesion amount of the slurry of the surface of the base material.
Meanwhile, in a method of supplying slurry to the surface of the base material, since a problem arises in that slurry nozzles are clogged, it is difficult to uniformly supply the slurry in the material width direction by using slit nozzles. For this reason, a method of supplying slurry to the surface of the base material by using a plurality of spray nozzles arranged in the width direction of the base material can be employed. However, even in this method, since the slurry cannot be easily uniformly supplied to the surface of the base material in the material width direction, the slurry is not uniformly supplied in the material width direction. In this case, the slurry unevenly remains on the surface of the base material even after the slurry is squeezed by the roll applicator.
Further, as a representative application defect in a slurry application process in the roll applicator type, there is known a stripe-shaped appearance defect called ribbing and occurring in a roll outer circumferential direction of the roll applicator. The ribbing is known as a defect occurring when a change in fluid pressure of a liquid meniscus between rolls of the roll applicator or between the roll and the surface of the base material (hereinafter, simply referred to as “between a roll and a base material”) exceeds a surface tension stabilization effect. Such ribbing easily occurs in accordance with an increase in amount (the liquid amount of slurry) of the liquid meniscus between the roll and the base material.
Additionally, as a related art of applying slurry onto the surface of the base material while suppressing an application defect such as ribbing and uneven application, for example, there is known a method of adjusting a slurry application amount (an adhesion amount) of the surface of the base material by ejecting a gas from slit nozzles to the surface of the base material in an inclined direction after applying slurry onto the surface of the base material (see Patent Literature 1). Further, there is known a method of adjusting an adhesion amount of slurry supplied to the surface of the base material by a roll applicator and adjusting the adhesion amount of the slurry again by ejecting a gas to the slurry of the surface of the base material using a gas wiping device (see Patent Literature 2).